This invention relates to a process for the synthesis of hydrous magnesium silicates.
The name "hectorite" has been ascribed to a natural trioctahedral smectite found at Hector, Calif., U.S.A. This clay is a hydrous magnesium silicate having the ideal composition Si.sub.8 Mg.sub.6 O.sub.20 (OH).sub.4 modified by having a portion of the Mg.sup.+ 2 and OH.sup.- ions replaced by Li.sup.+ and F.sup.- ions. The synthesis of hydrous magnesium silicates similar to natural hectorite has been described by Granquist and Pollack in "Clays and Clay Minerals" Volume 8 (Proceedings of the 8th National Conference on Clays and Clay Minerals) Pages 150-169. In the process described by Granquist gels of magnesium hydroxide and of silica are produced separately, are washed, are combined and are redispersed in water to form a suspension. Lithium hydroxide or lithium fluoride and sodium hydroxide are added to the suspension which is then treated hydrothermally by refluxing it with stirring until a product having a crystal structure similar to that of hectorite is formed.
While Granquist's product has a crystal structure similar to natural hectorite it does not have good rheological properties. A standard yardstick of rheological properties of a substance is provided by measuring the Bingham Yield Value of an aqueous dispersion of the substance. The term Bingham Yield Value (also known as Bingham Yield Stress, these terms being alternatives for the same property) is referred to in standard works on rheology for example in "Rheology Theory and Applications" F. R. Eirich (Acad. Press) Volume 1 (1956) page 658 and "Colloidal Dispersions" L. K. Fischer (N.Y. Bureau of Standards) 2nd Edition 1953 Pages 150-170 and "The Chemistry and Physics of Clays and other Ceramic Materials" 3rd Edition Page 463, A. B. Searle and R. W. Grimshaw.
The Bingham Yield Value may be determined by first obtaining a flow curve relating the shear stress to the rate of shear and then extrapolating the straight line section of the curve to the shear stress axis the intercept being the Bingham Yield Value. It can conveniently be determined on any viscometer capable of measuring a range of shear rates and shear stresses.
The product of Granquist, i.e. as a dispersion obtained using 2 g silicate and 100 ml tap water, gives a Bingham Yield Value of only about 15 dynes per cm.sup.2. This is a very low value, inferior to that given by natural hectorite. It also gives a low static gel strength.
Processes for the production of synthetic hydrous magnesium silicates having a crystal structure similar to natural hectorite but having better rheological properties than natural hectorite have been described in British Pat. Nos. 1,054,111 and 1,213,122. These processes describe the production of synthetic hydrous magnesium silicates having a characteristic hectorite-like structure and having varying contents of lithium and fluorine, by a process essentially involving direct coprecipitation of magnesium silicate.
The process described in British Pat. No. 1,054,111 involves forming a slurry by co-precipitation by slowly combining with heating and agitation in an aqueous medium a constituent providing the magnesium ions with constituents providing the silicon (as silicate) hydroxyl and sodium ions and treating the precipitate hydrothermally. The concentration of the slurry is desirably such that the concentration of the product formed is from 1 to 8% by weight, preferably 4% by weight. The hydrous magnesium silicate contains fluorine and lithium.
The process described in British Pat. No. 1,213,122 involves precipitating a magnesium silicate by combining an aqueous solution of a water soluble magnesium salt with an aqueous alkaline solution of one or more sodium compounds in the presence of dissolved silicon compound and hydrothermally treating the precipitate under pressure. The concentration of the precipitate is preferably not more than 5% by weight. The hydrous magnesium silicate product contains no fluorine and optionally contains lithium.
The products of the processes of the two British patents are also characterized by providing dispersions giving exceedingly high Bingham Yield Values in excess of any known to be given by natural hectorite dispersions and usually above 40 for example from about 50 to 250, dynes per cm.sup.2 as a 2% dispersion in tap water, and high static gel strength. Such products have found widespread use by reason of their excellent rheological properties.
The above described coprecipitation processes are subject to certain difficulties of operation. If operated using a relatively high excess of alkali over that required for the reaction on a stoichiometric basis viscosity problems may cause difficulties in mixing the reactants and this may result in a lowering of the yield.
Operation of these processes at a relatively high concentration may cause filtering and washing difficulties and operation at low concentration results in the necessity to handle large quantities of liquid. This results in higher heating costs and higher equipment costs, particularly in relation to the hydrothermal treatment stage of the process.
It may be difficult, regardless of the concentration and excess of alkali used to produce hydrated magnesium silicates which disperse readily in cold water to give absolutely clear dispersion. Such properties can assume importance if the hydrated magnesium silicate is for use in for example the toiletries industry where the appearance of products containing such silicate is important.